1. Field of the Invention
The invention relates to a method for the making of heteroepitaxial layers, and more specifically to a method for making of at least one thin layer of a semiconductor material of one type on a semiconductor substrate of a different type.
The invention relates to the field of "thin layers" and particularly to the field of monocrystalline thin layers epitaxially grown on a substrate of a different nature.
The invention shall be applied, for example, to the growth of GaAs or InP layers on Si, and can be used for the elimination, by blocking, of the dislocations generated at the crystal/substrate substrate.
2. Description of the Prior Art
During the heteroepitaxial growth of GaAs on Si, dislocations are generated at the crystal/substrate interface and get propagated in the thin layer during the deposition. Very schematically, the presence of these dislocations is due both to the difference between the lattice parameters of Si (0.54 nm) and GaAs (0.56 nm) and to the difference between the heat expansion coefficients (2.3 10.sup.-6 .degree. C..sup.-1 for Si as compared with 5.6 10.sup.-10 .degree. C..sup.-1 for GaAs).
These dislocations, once nucleated, are practically impossible to eliminate during a normal MBE or MOCVD type growth, which considerably limits the field of application of the heteroepitaxial processes of GaAs on Si.
For, the dislocations acting as recombination centers, drastically reduce the lifetime of the minority carriers. The result thereof is that it is practically impossible to make bipolar type components, such as lasers and photodiodes in heteroepitaxially grown layers of GaAs on Si.
A method for blocking of the dislocations has been devised (cf. the French patent No. 88 044 38) filed on 5Apr. 1988). This method can be used to obtain layers practically free of defects. The principle of this method (known as a method of forced growth) is shown in FIG. 1. One of the drawbacks of the method is that it calls for the use of two dielectric levels and two masking levels to make, firstly, the seeding bands and, secondly, the bands enabling the access of the gas of the forced epitaxy phase (see FIG. 1).
The method described in the French patent application No. 90 12 443 makes it possible to provide for only one masking level. FIGS. 2a to 2f show a manufacturing method described in this patent application. This method is used, for example, to make a monocrystalline layer of GaAs on silicon without defects. First of all, therefore, a layer 2 of GaAs is made on the silicon substrate (FIG. 2a).
A layer of GaAs is covered with a layer 3 of Si.sub.3 N.sub.4 (FIG. 2b). Apertures 4 are etched in the GaAs layer (FIG. 2c). Through these apertures, a selective chemical etching operation is carried out on the GaAs until all that is left is a seed 20 of GaAs shown in hatched lines in FIG. 2d. Finally, GaAs is made to grow by vapor phase epitaxy from the GaAs seed between the silicon substrate and the Si.sub.3 N.sub.4 (FIGS. 3e and 3f). The dislocations are blocked on the substrate or on the layer of Si.sub.3 N.sub.4 (FIG. 3f).
According to one variant of the method described in the U.S. patent application Ser. No. 90 12 443, after the making of the layer 2 of GaAs, bands 8 (FIG. 3) are etched and then the entire piece is covered with a layer 3 of Si.sub.3 N.sub.4 (FIG. 4). Then bands 9 are etched in the layer 3, perpendicularly to the preceding bands 8 (FIG. 5).
Through the apertures 6, made by means of the etching in bands 9, the GaAs is subjected to selective etching until seeds 20 (FIG. 5) are left. Finally, the vapor phase epitaxy of GaAs is carried out, and there are obtained blocks of GaAs on silicon (FIG. 6), the dimensions of which can be predetermined when the etched bands 8 and 9 are being made.
This method thus makes it possible to achieve growths with semiconductors of different natures and to obtain heteroepitaxial structures in which the different layers are nevertheless crystalline and have neither dislocations nor defect planes. Furthermore, this method enables the reducing of the cooling constraints due to the differences in heat expansion coefficients between GaAs and Si.
This method provides for the use of the surface of the original substrate as one of the confinement surfaces of the directed lateral growth. The technique is thereby greatly simplified, for it requires no more than the implementation of a single deposition step (the deposition of an upper layer of Si.sub.3 N.sub.4) and of a single step of selective epitaxy (the step of forced growth itself).
However, in this method, problems may appear during the vapor phase growth, especially when one of the directions yy' or xx' (FIG. 5) corresponds to a &lt;011&gt; type crystallographic direction of the plane (100) of the substrate. Indeed, the appearance of lateral facets is observed, and these facets prevent the growth from occurring normally.
FIG. 7 illustrates this situation in a top view. The seeding bands 20 are oriented along the direction &lt;011&gt; of the plane (100) of the substrate, and the facets appearing after growth are shown with their indices in FIG. 7. The phenomenon is inconvenient for it makes it difficult to obtain rectangular blocks after growth.
It is an object of the present invention to overcome this drawback.